Unfortunately the GTEx images are generated on demand then erased later. In case it helps, these are dynamically generated by R from the track data. We can help with finding the exact parameters if you are interested.
The temporary nature of these images is noted by the 'trash' directory they exist in that's part of their URL:
For example that link no longer works. Another option if you have only a finite number of genes you are interested in you can save the images locally.
If you have any follow up questions, our public help desk can always be reached at firstname.lastname@example.org. You may also send questions to email@example.com if they contain sensitive data. For any Genome Browser questions on Biostars, the UCSC tag is the best way to ensure visibility by the team.