telling us which template you are using would also help. You are talking about this poster, but the forum is about all the posters provided on latextemplates.com. We don't know (yet) which template you are using.
The smart way: Calm down and take a deep breath, read posts and provided links attentively, try to understand and ask if necessary.
Check with overleaf though, if they're using UTF8 or something else for their input encoding (i.e., the encoding the .tex file gets saved under).
I've read somewhere somewhen, that in combination with Polish, Cork encoding should be used. I forgot the particulars about that, though---it might (no longer) be necessary.
Secondly, I'd skip loading the polski-package. Seems to me that babel with Polish as main language should already do its job.
Should that not help, try something smaller first, like a standard article class, inputenc, (fontenc,) and babel packages---and just a little bit of Polish text; perhaps then you can pinpoint more closely what it is that's actually wrong.
RXN wrote:
One more thing... I added .jpg file and it still says it can't see it
I used Add files menu
Should I add it somewhere else?
it would be better, if you started a new thread for a new question, otherwise this thread might get out of hand---in this case, Graphics, Figures & Tables sounds like a good place for this new question.
However, you need to put \includegraphics[<options>]{<graphicsfile>} in your document, where this graphics should appear, provided, graphicx package is loaded.
Come to think of it, refrain from using Polish characters in the file name; not even space is a good idea (most shells separate arguments with space); I usually stick to A--Z, a--z, and hyphen (-).
(I don't need a period (.), because I let LaTeX decide, which file to load if more than one is present with different extensions)