1. Open in new tab: https://www.roblox.com/Sell-Top-item?id=465414855
2. Open new tab and go here: http://asset-markotaris.rhcloud.com/
3. Drag the "Download ROBLOX Asset" Button to your bookmarks bar.
4. Go back to the Sell Top page.
5. Click on the bookmark you just added
6. You should get a big, high quality template of the shirt.
7. Right click Save Image as...
8. You know the rest, upload to your group and then your done. =)