How can I create a photo backdrop?

It will depend on what you want it to look like. My first instinct would be to use a flat sheet. You can purchase in a color of your choice or white then paint or tie-dye.

Mariya....I may be off base on understanding what you want but I have a large empty frame and I use small roping or yarn and staple it to the back of the frame using about 3 rows (according to size pic you want to use). I purchased some tiny colored clothespins and clip each picture with one or two. Looks real cute even if you prop it against wall or table leg. Hope this helps.

Marilya: maybe this will help

https://expertphotography.com/diy-photo-backdrop/

I made a sample back drop for Home talk for New Years on the nightly news one year! That was a blast! Here is what I did https://www.homeroad.net/2013/12/mirrored-new-years-eve-photo-backdrop.html#more

You could use a plain sheet or tablecloth