I'm not quite sure what you actually mean, since
poster to me means a single large sheet - so if the graphics are already given, which was another question, the job was mostly to resize the graphics to fit the format.
If slides of a presentation are meant, then the quickest and easiest way to create a presentation I know, is as
Espionage724 already said, using LibreOffice Impress ("Powerpoint").
There are also some other ways/progs to create presentations, like there are some LaTeX packages providing different ways for creating presentations, but if you are not already a well versed LaTeX user I wouldn't recommend that way for this project, since I guess there is not enough time to learn several tool plus LaTeX, plus learn how to make a good presentation slides, plus create the slides.
Old engineering wisdom:
You either do a new job with old tools (already well versed on).
Or you use new tools on an old job.
But never do a new job with new tools.
So, you better use a well versed jobs as a chance to learn new tools, to enlarge your toolbox.
Graphics:
If the graphics are given, they are hopefully at high resolution or even better as vector graphics. The latter ones can be down- and upscaled without quality loss. Raster graphics (.jpg, .gif, .bmp,...) can only be downscaled at good quality.
To enlarge low resolution pictures Gimp provides a lot of tools to adjust picture quality. But don't demand magic. Besides the tools are not easy to use for newbies, enlarging a raster graphics means adding pixels - filling the gaps of missing information. You could try some AI based tool, but missing information stays missing information. All also any AI can only do is
guessing what would fill the gaps between the pixels.
The "onboard" graphics editor of Powerpoint/Impress is more than quite sufficient enough for producing own graphics for presentations. Otherwise Inkscape was a good choice (question is again, which tools you already know to use.)
Anyway, the pictures you are showing are good for some poster, but they are (very) bad for a presentation - way too much information density; impossible for any audience to grasp while a presentation runs; except the slides are not meant to be shown while a presentation, but to study for anybody for themselves. Anyway I hope that's not the only quality you have, but there is (way) better, because they... - frankly: they suck.

Nobody is going to give those blurry things a closer look.
The precept for presentation's slides is:"Less is more. The less, the better." Because the slide show supports the speaker, not replace her/him, and the audience neither want's to read a book, nor watch a video, but listen to what the speaker has to say.
Enjoy
Don McMillan to learn about how to
not make PP presentations.