Espen A

Its not really worth the time getting this perfect, but now im aware next time i shoot.
So i just went with the easy solution on these images. As always thanks for support.
Such a great place to ask questions.😀 

How did the second document come to exist? Did you create it by choosing "New from Clipboard" or did you create a new document first and then paste in the stuff you copied from the first image?
If you created the second document by choosing "New from Clipboard," then I agree that the transparent space is baffling. However, if you created the second document beforehand, you would have created a canvas of a given size (in your case, it is 4121x3067 pixels). When you paste in the copied stuff, you will not automatically resize the pasted pixels to match the canvas; the pasted pixels will keep their original pixel dimensions. In this case, since the second document's canvas is larger than the pixels being copied and pasted, empty space has been left around the perimeter.
You could partially solve this problem by choosing "Clip Canvas" from the Document menu after doing the Paste. However, since the pasted pixels do not form a perfect rectangle, there will always be some space around at least one or more of the edges.

Is it possible there are stray, semi-transparent pixels included in the selection causing the extra space?

My guess is that Brian_J could be correct.
The Selection Brush Tool can pick up ‘stray’ pixels and the ‘marching ants’ only show the portion of the selection where opacity is 50% or greater. Therefore any pixels with opacity of less than 50% will not be included within the ‘marching ants’ boundary but may be included in what is copied.
In this case, you could try using the Rectangular Marquee Tool instead but you will lose some of the frame as it is not fully ‘squared’.
There are other methods of selection but they are a bit more lengthy so see what you can get with what you already have before asking about them.

You should be able to straighten the image in various ways.
You could try using the Perspective Tool and see how that goes – maybe use some guides to help with the positioning.
There will be other methods so if you can share the original image in public we can experiment and see what’s best.
(If it is a lens distortion then I’m not the best person to be advising on that.)

I'm not sure how the selection was made, but you could use the Pen tool to create a curve that outlines the subject mater of your photo. Give the curve layer a fill color, then Ctrl-click (command-click on Mac?) to select the curve, copy the photo layer, and paste it into a new file via 'New from Clipboard.' Creating the selection in that manner would ensure what you paste into the new file is only what you want ... i.e., no space around the subject matter.
Then you could do any required straightening in the new file.

Create a group of all the curves then save the group as an asset
You can always ungroup it again whenever you drag it from the assets panel

Thanks for sharing. Found some nice fonts😀

Thanks Dan
Problem solved 😀

Hello,
A feature that I would like to see in Affinity Designer is that of a "color scheme generator" of sorts - or at least a color guide that displays color harmonies (with tints and shades of each color). This guide would let you pick a color, and then would generate colors in different categories that would go well with that color (i.e. analogous, complementary, split complementary, etc.) If this is not yet available, I think it would be really helpful.
Thank you!

Thanks for your file, I can see from your Navigator and Layer Studio icons that these lines haven't been successfully removed from the image - and when opening your file here I'm seeing the following -

Please try using the Flood Select Tool to select the White areas of the image, now use Select>Invert Pixel Selection to swap your selection to the black bars. Once you've created your selection use Edit>Cut to remove the selection area from your image. Now export again, ensuring you use the correct export options.
See my below screen recording showing this process  
2020-07-02 15-04-11.mp4

As an alternative, you can create a large rectangle, then create a grid of lines over the rectangle.
You can then select all of the lines, use Expand Stroke, then use the Geometry Divide function to create a grid of squares.
One issue with this is that the resultant squares will be slightly smaller than the ‘grid’ you made with the lines, but that might not be a problem in some circumstances.

See:
https://affinity.help/designer/en-US.lproj/index.html?page=pages/ObjectControl/duplicate.html?title=Duplicating%20objects

You can create a square more quickly with the Rectangle Tool - see the blue square below the Pen Tool on the tools bar.
Hold down the Ctrl key and drag an object to duplicate the object. If you are creating a regular offset between duplicated objects, then "power duplicating" can speed up the process: after the first duplicate and drag, do not deselect the dragged object, and then repeatedly press Ctrl+J to create a series of offset duplicates.

This worked great. Using the curve shape and changing the blend mode to keep the wood texture 😀
Thanks. 

Welcome to the forum, 
You can use the pen tool to make accurate shapes, fill them, and then use: Select > Selection from layer and delete, this creates a selection and deletes the curve you have made to create the selection.
Alternatively you can just use the curve shapes and change the blend mode to something like colour if you want to keep the wood texture.

Thanks so much. Will try this asap.