Can't UNITE in PATHFINDER & put a OFFSET PATH on images

[Atiya22467658xmf9]

I am not Illustrator saavy so the responses to similar questions haven't helped me. I was able to unite images through the Pathfinder tool panel and then put on a offset path to create an outline. I logged on to Illustrator today and I am no longer able to do the same thing. I only get an error message that says "The filter produced no results. Please select two overlapping paths." I have tried restarting, deinstalling/reinstalling, copying onto new sheets and selecting Gap Detection in the Shape Building Tools Panel. Nothing is working.

[Monika Gause]

Did you use the Pathfinder panel from Window > Pathfinder?

If not: use it.

If you already used it, then please show screenshots.

[Atiya22467658xmf9]

Yes, I did.

I opened my file, copied it and pasted it in front.
Then, I selected all files again and clicked on the Unite button on the Pathfinder panel. That's when I get the error message.

[Atiya22467658xmf9]

[Larry G. Schneider]

Everything shown are listed as images the Layer panel. What you are trying to do cannot be done to raster images. 

[Atiya22467658xmf9]

I did it earlier and it worked just fine, but now stopped working. I've been able to do it again recently with other images, but can't tell which images will work and which won't and why it worked before and now won't.

[Monika Gause]

As Larry said: it doesn't work with raster images.