A rentry mask is a template that you can clip images to so they look more interesting rather then just having a basic image on it's own. The picture clip to it is an "overlay". All together, it is called a graphic.
This is a mask by itself.
(made by serrnz on pinterest)
This is a mask with an overlay clipped to it (graphic)
How to clip an overlay to a mask:
Find a mask you want to clip an overlay to (you can google 'rentry mask' or 'rentry overlay' to find one - you can use pinterest too)
Go onto ibis paint and create a new canvas, then import your mask
When you import the mask a popup with 'Extract Line Drawing', 'Background Removal' and 'Cancel' will appear
Click 'Extract Line drawing'
After you have done that then import your overlay but this time when the popup appears click 'Cancel'
Then click the arrow on the menu to clip the 2 images together
When you're saving it, remember to 'Save as Tranparent PNG' so that when viewing the rentry it looks normal on both dark and light mode
when inserting the graphic onto your rentry, you can use the script: {300px:300px} so that when you put it in it wont be too big (the 300px:300px is the size and you can change it)
you can also create another canvas thats 200x200 and import the graphic, then save as transparent png again if you're planning on letting someone else use the link to the graphic so they don't have to change the size (it may be a little pixelated)
Extra masks/overlays I found: