My company has just changed its visual identity and I have updated the basemaps with the new logo.
This one was provided to me in .jpeg, with a high resolution (300dpi) and in several sizes. Whatever the version, when exported .pdf the drawing, the logo is ugly, crenellated.
I searched and modified everything I could find that could directly or indirectly influence the resolution of this image in the saved pdf, without much improvement.
Normally if you were your background and if you go insert==>image, it asks you for low resolution or high resolution. Try with high which worked very well for me.
You may have clicked a previous time on Don't show again, you should go to tools==>options==>messages/errors/warnings and then check select the resolution