Best answer by Mathis
We've received reports about replacing images on the platform not working properly last night - this has now been fixed and I can only assume that this has fixed initial uploads as well.
If image uploads don't seem to work during the upload, you can also try to upload the artwork in the track's edit mode after the upload has finished and you've saved (after clicking on the small "pen" icon under the waveform), or directly on the track page by scrolling over the field where the artwork would show an click on "Upload image". If one way to upload it fails, another way might still work (depending on what's broken).
Also, quick note about your recent uploads: Before sharing to Facebook and Twitter, make sure to run the track URL through their debugging tools to make sure they fetch the current image when sharing.