Michele, the term “tube” has been used in different ways, and mean something different.
“Picture tubes” is what PSP is using. Those images that you can apply like stickers (click, click, click), or draw with, like ropes, chains, and all those you can see in my store for example.
“Tubes” is a term that describes “images without a background”. This is NOT typical of PSP as you can create those in Photoshop, or any other program. They are most often in png format (with transparent background). Those “tubes” can be just anything, like objects, animals or people. They are extracted from a jpg image using whatever tool is needed and then saved in png format. PSP users often THINK they need to convert them into .psptube format to use inside PSP with the Picture tube tool, but it is not the case. I have an article here that explains a bit more.
Hope it helps clarify that terminology.