Can I use <img>tags in a description of a product (or how could i make this possible?

[schoolboy]

This is basic HTML path structuring - not a zencart issue - so you look like you need to familiarise yourself with the basic HTML protocols of setting absolute and relative paths to whatever resource you are trying to parse.

Essentially, you can fill your textual inputs with almost any "standard" html and it will render faithfully in the browser. But there are considerations of course, and these relate to rendering remote content (is the remote server reliable) to things like pagespeed, http vs https ...

Far better to have all images on the same server as your site.

[mc12345678]

Thanks!
I'm going to use this way. Currently in proces of checking and where needed changing the existing url's... lots of work.

As noted earlier there are tools that can be used to make changes to many items/product at one time instead of one-at-a-time.