How to find promotional images
Promotional images must come from official sources, so there are two primary places to look for them: the game's official web site and developer- or publisher-controlled store pages, like Steam, Google Play, or Apple's App Store.
Getting images from Steam
A game's Steam store page usually includes a number of official screenshots that can be submitted to MobyGames. First, make sure you're on the game's main store page, like this one. Elsewhere on Steam are screenshots taken by users, but those aren't what we want.
Once you're on the game's Steam store page, you'll see a gallery of screenshots at the top. If you click a screenshot, it will open up into a larger version with a "Download full-size version" link in the upper left corner. Right-click that link and save the image to your computer, and repeat the process for each image in the gallery.
Getting images from Google Play
Android games usually have a number of screenshots on Google Play that can be contributed to MobyGames. Visit the game's page on Google Play to find them.
Click an image in the gallery to make it larger, then right-click and save the image to your computer. Repeat for each image in the gallery.
How to contribute promotional images
Now that you have the images, find the game on MobyGames and click the green contribute button in the upper right corner of the game's page. Pick "Promo Art" from the list on the contribution page.
If the game you've selected already has some promotional art, you'll be presented with a list of image groups to contribute to. Pick the top option, "new group of images", and press continue to get to the upload form. If no promotional art has been submitted for that game yet, a new image group will be created automatically.
To upload the images, you can click the "Select Files" button and find them on your computer, or else select them on you computer and drag and drop the images onto the "Select Files" button. Up to 20 images (for a total of at most 20 megabytes) can be uploaded at a time.
Now, you'll fill in a name for the group. Pick a name that describes the source of the images, like "Steam Store" or "Google Play". In the comments box, you should put a link to where you got the images. Once you've filled the boxes in, click "Save" at the bottom of the page, and the images will upload.
Once the images have uploaded, you'll be shown a list of the uploaded images, and you'll have the chance to select the specific image type, reorder the images, add any official titles or captions the images had, or add extra comments. "Screenshot" is the default type, and screenshots from store pages usually don't have captions, so you probably don't need to do anything here. Just click the "Continue" button in the upper right corner of the page.
On the final page, you can add any notes you might have for the approver who checks your submission. If you didn't include a link to where you got the images in the group comments, you should mention where they came from here. Otherwise, you're done! Just click the "Continue" button one more time, and the images will be submitted for approval.