What is a good website to create product screenshot mockups that allow gif uploads?

In general, the site is pretty slow (takes long to upload screenshots). Furthermore, it has many usability issues, that make working with the tool very tedious. You can for example only upload 5 screenshots per project which is not enough if you support different languages or simply want to make more than 5. Another missing feature is the possibility to copy the styles for the next screenshot. At the moment, you cannot even upload a screenshot which makes the tool completely unusable. In other words, you pay for a tool that you cannot use because the most important function is buggy.