This post is almost not relevant any more due to the fact that we all will be pushed into the make.powerapps experience in “the fullness of time”, but I will still be using the classic viewer for the foreseeable future 
From time to the images can get distorted due to unknown reasons. This is not a big deal as the buttons still work, but can be annoying for the users. You might have seen this in the 2 places:
Advanced find:
Form configuration:
Fix
There seem to be several ways to solve this issue, but not all seem to work the same. I have seen some options describing a cache clear in the console of the browser, but for some reason none of those worked for me. Lately I have had to open the developer tools and delete a bit more manual.
Click F12 in the browser where you see the error
Open the Application tab on the dev tools
Clear or delete anything within the storage related to dynamics.
Try to restart the browser, and hopefully that should do the trick:)