Print services accept nearly anything: a phone photo, a screenshot, a 2 MB JPEG that has been through three messaging apps. They warn you about resolution and then print whatever you send. The result is on you, so here is what to check before you upload, in the order it matters.
Resolution, and the viewing-distance rule
The number people quote is 300 pixels per inch, which is right for a print you hold in your hand. It is more than a wall print needs. A poster seen from a metre and a half looks sharp at 150 pixels per inch, and a large canvas across a room is fine at 100. Divide your image’s pixel width by the print width in inches to get the figure. A 4000 pixel wide photo printed 60 cm (about 24 inches) wide is 167 pixels per inch, which is plenty for a wall.
What does not work is enlarging a small file and hoping. If a photo is short on pixels, an upscaler that rebuilds detail will get you one size up convincingly; two sizes up starts to invent texture. Better still, go back to the original file rather than the one you shared.
Colour: keep it in sRGB unless the service says otherwise
Almost every consumer print service expects sRGB. Files in Adobe RGB or Display P3 will print, but the colours shift, usually toward dull, because the service converts them on the way in. Export in sRGB and embed the profile. The exceptions are the labs that publish their own ICC profiles, such as Saal Digital; there you can soft-proof against the actual paper, which is the closest thing to seeing the print before it exists.
Prints also look darker than screens, because a screen emits light and paper reflects it. If your monitor is bright, your prints will come back moodier than you remember. Turning the monitor down to a normal room level while you edit fixes most of that.
Crop to the size you are ordering
A camera frame is 3:2, a phone is 4:3, an A-series poster is 1:1.41, a 50 by 70 poster is 1:1.4, a canvas is whatever you order. If you do not crop, the service does, usually from the centre, and a head disappears. Crop in your own software to the exact ratio of the print, check the edges, and if it is a canvas leave extra image around the outside for the wrap.
Sharpen for the size, at the end
Sharpening is the step most people skip. Downsizing a big file to the print resolution softens it slightly, and printing on matte paper or canvas softens it again. A modest sharpening pass on the resized file, applied once as the last step, brings it back. Glossy paper needs less, canvas needs more. Do not sharpen before you resize, and do not sharpen a JPEG that has already been sharpened, because the halos add up.
Save it properly
Export as a JPEG at the highest quality setting, or a TIFF if the service accepts one and the file is not enormous. Turn off any “optimise for web” option, which strips the colour profile. Name the file so you can find it again; the service will keep it for a while, but the next reprint is easier from your own copy.
The five-minute version
Use the original file. Crop to the ordered ratio. Check pixels per inch against the viewing distance. Export sRGB with the profile embedded. Sharpen once, lightly, at print size. That handles nine prints out of ten; the tenth is a very dark or very saturated image, where a test print at a small size before the big one saves money.