@m1lk-jpg Hi! I will try to explain the simplest way to do it that I have found, but I exclusively use photoshop so if that’s not your program perhaps someone has something to add, if so, please feel free!!

1. Save the overlay gif that you want to your desktop as a gif. I post the ones that I create here, but this is the most popular “falling glitter” one that seems to be almost universal.

2. Open the “base” image and the gif in photoshop.

3. Go to the gif, and make sure that your “timeline” editor is open. (Window > Timeline)

4. Convert the timeline to a video timeline by clicking the button on the bottom left of the timeline window that sort of looks like a stacked “volume” bar symbol on its side. This is different than the frame-by-frame timeline. Once you’ve done that, select all the layers.

5. Go to Filter > Convert for Smart Filters. Say yes to the dialogue box. You should now be able to click the “play” button on the timeline and see the gif sparkling as one seamless layer.

6. Now the fun part! Take that singular gif layer, and drag it to the open window with your still image. It should just sit on top in a black square at first. Adjust it to be the size you want, or if you don’t want the gif to stretch, once you’re comfortable with the process you can always play around with duplicating the layer and tiling it! 

7. Now you get to play around with effects. To “overlay” the gif, play around with setting the gif layer to “Screen,” “Lighten,” “Color Dodge,” or anything else in that little subsection of layer styles. Basically, this tells that layer to only show the pixels that are lighter than the image, meaning that the black will be erased and you will see your dust/glitter “floating” on the image! It will depend on your image and preferences as to which style you choose, I sometimes even use two or three styles in one gifset to match the levels of intensity, as you’ll find some are too “bright” on darker gifs, some almost disappear on lighter gifs, etcetera. 

7. In the timeline window, tell it to create a timeline. You should now see two layers: your gif, and your image. On the timeline, you will see a slider like this:

We are currently focused on the GREY slider that is sitting around the black layer (the gif) like a hamburger bun. See how layer 5 continues, but that slider only encompasses the length of the gif? That tells the gif to repeat at that point. It is likely that when you create the timeline your slider will end at the end of your still image, so all you need to do is drag it back to be at the very end of the gif like shown above! Otherwise your animation will end and you’ll have seconds of still image before it loops again.

NOTE: Once you get more advanced, or if you’re going to edit your image further after creating the timeline, you’re just going to want to make sure that your “sandwich” is neatly stacked, sometimes if you paste more images in or make a text layer, they will start wherever that red bar is, so you will need to drag them back to make sure they are at the beginning. It’s a good idea to play the gif in preview before posting it to make sure everything’s set!

8. You’re ready to go! Save the file as a gif, and make sure that it’s set to repeat “forever” instead of “once” in the save dialog. Tumblr gifs need to be under 3MB.