Guy who’s totally not a designer needs some design advice | Graphic Designing

Hello there, people who actually now stuff about graphic design, unlike me. My background is STEM.

I’m just a guy trying to make the white walls in his apartment less boring. So I “made” a few gaming-inspired posters (and by made I mean threw together pre-existing stuff he found online in hopefully harmonious ways), which I intend to get printed on A2 size high-quality posters (16.53 x 23.39 inches). These posters will be in portrait orientation, not landscape (I hope those are the right terms).

In the printing shop’s website I found a list of requirements, that according to google are pretty standard, but I’m struggling with a few of them. Here’s the list and my situation regarding each point:

• files must be either JPEG, PDF or TIF

Going with TIF because JPEG is JPEG and PDF just seems silly to me. * files must be at least 150 DPI

Considering how the vast majority of the source material I used was in “landscape” resolutions, unlike my intended “portrait” final result, and not very high resolutions at that, I’m just going for the minimum 150 DPI. Based on what I found online, 150 DPI A2 size means 2480 x 3508 or 3508 x 2480 pixels, so I’m using that resolution. In the vast majority of cases this meant resizing my images to make them a bit bigger. * files must be in CMYK color space

Ok I’m familiar with color spaces, RGB and HSV and whatnot. However I wasn’t aware of the software’s limitations regarding this stuff, or of the possibility of having colors shift during conversion. I made all my stuff with the software I’m most familiar with, Paint dot Net (I’m a casual, I know), and from what I gather online, converting to CMYK requires either using Adobe products (that’$ not gonna happen) or GIMP with plugins, which is probably what I’ll do? I’m still confused and hesitant in regards to the apparent issues with colors shifting. * the images shouldn’t contain any important details or information within 3mm of the edges

… okay? Seems straightforward enough. * files must have 3mm of bleed margin on all sides, plus crop marks and ideally registration marks

… what? This one is really tripping me up. I understand what these things are after googling, I just don’t understand how adding this bunch of extra stuff to all my borders will affect the final print result, or how I should add them to begin with. Do these fancy printers used to print large format stuff always print things in bigger paper than the final intended result, which is then trimmed down? It feels counter-intuitive to someone who’s only ever printed stuff in standard printers, the kind you have at home. In those printers, if you try to print something that’s bigger than the paper you’re using, there will either be cropping or shrinking of the source image. How different is this from “fancy” professional printers? Can I make my files wider and taller than 150 DPI A2 and trust that the excess margins will not result in any shrinking of my desired output?

For illustration, here’s one of the image files for illustration purposes (wow, imgur really degrades the quality of stuff you upload huh?). All the other ones have file sizes above 10MB so I’m not sure where I would host those.

So, could any of you shed some light on this?