Can you recommend any simple open-source tool to add captions to GIF images, for example a tool that would allow to create a meme?
Alternatively, a non-open-source but still free tool would be good too.
The tool should be as simple as possible (no Photoshop-like tools like GIMP), for example something like MS Paint (or kolourpaint, which is an open source alternative) that just shows the GIF image (or its frames) and allows to insert a textbox that would be visible in all frames of the GIF image.
There are plenty of web apps and online tools like this on Internet, but apparently it's very difficult to find a tool that can be installed locally and used offline.

