perspective/road position on this photo is noticeably different than on other (yes, I know how hard is too take matching images)

I agree with both of you. Can you find a picture that uses the same perspective but clearly shows some compacted surface?

What about one of these three pictures?

https://commons.wikimedia.org/wiki/File:Track_in_Schwinkendorf_2021-09-07_21.jpg
https://commons.wikimedia.org/wiki/File:Dirt_track_leading_into_Safari.jpg
https://commons.wikimedia.org/wiki/File:Dirt_track_-_geograph.org.uk_-_7486405.jpg

Here is a mockup with the first file (color adjusted):

argh, sorry to complain but this one looks quite sandy to me on tiny thumbnail (note: I was similarly complaining when I was selecting pictures on my own)

curiously, it is not a problem when I look at https://commons.wikimedia.org/wiki/File:Track_in_Schwinkendorf_2021-09-07_21.jpg

and maybe it will not be a problem given that description is there?

How about this one? It's the same track, only at a slightly different position. Due to the shadows cast by the trees, the surface doesn't look so yellow / sandish-ly, while the grass makes it clear that it is not paved:

https://commons.wikimedia.org/wiki/File:Track_in_Schwinkendorf_2021-09-07_20.jpg

By the way, this, also the same track at a slightly different position, maybe use to replace the current grade5 pic?:

https://commons.wikimedia.org/wiki/File:Track_in_Schwinkendorf_2021-09-07_04.jpg

By the way, I don't find the current picture for grade4 so good. On the thumbnail it looks much closer to grade5 than to grade3, even though it should be in the middle.

Great.
Here is a commit with the two links you posted.

Here is a screenshot in the Android simulator:

I have other things to say about the pictures (too small! hidden by text! who cares about the sky! grade1 season is confusing! irrelevant water puddles on grade3!). But at least it's a step forward.

Yes, about the text, I think this should be changed to text below the picture, not on top of it. For all quests. But yeah, that's a different ticket, and kind of currently blocked by a WIP PR.

Regarding "too small", I thought about being able to show a larger version of the picture by tapping on it (another time after selecting it), but that would be a major work, as that should then work for all of the pictures. It would also blow up the app size a lot, making it maybe worth it to think about not bundling those resources in the app but downloading+caching them after app install or something. Anyway, that's a bit more blue sky, and also anyway blocked by a WIP PR / near-future refactorings.