Hello,
Since I updated to the latest GH update the image quality of the thumbnail is of noticeably lower blurry quality. I've had this problem before but fixed it with a fresh install of GiP 2.90, and even now it fixes the problem until I update GH.

I did another fresh install of GiP 2.91 and I'm still getting poor quality thumbnails in the radio downloads, not TV. I haven't changed any settings or defaults and I select all the various components during the installation.

Any reason as to why this keeps happening or how to get a higher quality thumbnail image in the .m4a file?

Use for larger thumbnails.

I've had a minor similar, but possibly unrelated, problem for a few months. Thumbnails for all BBC Local radio programmes I've downloaded (eg Ralph McLean - Rock & Soul http://www.bbc.co.uk/programmes/b038c4sb, On The Wire http://www.bbc.co.uk/programmes/p001d75v) are distorted. Thumbnails downloaded separately (get_iplayer --force --type=radio --thumbnail-only --pid=) look fine though, as do Radio 1-6 progs.

If there's no apparent solution, is it possible to designate the thumbnail for a radio file manually, in 'Properties' or similar?

PS my get_iplayer is the latest version 2.91.

Those thumbnails look fine here. I suspect whatever you are using to view thumbnails is doing the distortion. Post a screenshot of what you're talking about.