Since the images are sent separately from the text, create your images with a convenient vector application (Draw is a good one). Save them in .odg format (if under Draw) for future edits.
When you’re ready to send, export the files as .svg. This is also a vector format which will allow lossless image manipulations, while TIFF and PNG are raster formats (= bitmaps), which will cause distortion or pixellation when the images are adjusted.
If you are using Gimp for the conversion, first scale your image with
Scale & Size. This is where you can set image resolution.
As an example, the image below was initially a .png. When opened in Gimp, it reported at 96 dpi (which is supposed to be the “standard” screen resolution nowadays). I changed the resolution to 300 dpi and simultaneously applied a 300% increase factor to the size to compensate for denser dots. 300% is not exactly 300/96 but this was a quick’n’dirty test. If you start from a .svg, there will be a better overall quality.
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