These days, few pay attention to file sizes. But doing so still makes a lot of sense. And most images can be cleaned and scaled to work better as textures or background images. There are many ways of doing this, but as a general rule for simple plans with few colours, an indexed .png or .gif will give a smaller (and clearer) background image than a .jpg. Here is my approach when I need a background image - using the GIMP (but other image software can be used too):

1. Import image - often using Screenshot and Edit > Paste as > New Image - to the GIMP.
2. Select part of image you need and use Image > Crop to Selection.
3. Image > Scale Image to a size where it is still usable (usually well below 1000x1000 for a background image - depending on the size of the plan, of course).
4. Colours > Posterise to a point where the colours I want are clear, usually below 16 colours, depending on the original. (To get a B/W image, try Colours > Threshold instead of Posterise.
5. Image > Mode > Indexed reduces the size significantly.
6. File > Export using .png extension.
   [/list=1]At the same time, I use the opportunity to remove unwanted information, maybe my own text, ensure the correct rotation etc. etc. When this image has been imported as a background, I end up with a .sh3d file where the (each) background image usually is less than 50 KB.
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