Mentimeter supports four different cases where you can upload images:
Attaching an image onto a question slide.
Answer options for Multiple Choice question type
As a background image for your own themes (paid accounts)
The general rules for uploading images are as follow:
Supported formats
The image formats that are supported for your Mentimeter presentations are: JPEG, PNG and GIF.
Maximum size
The maximum file size is 15 MB.
Recommended dimensions
Question images, images on Content Slides, and theme background images will look best in an aspect ratio that is as similar to your presentation screen ratio as possible, but feel free to try images with other dimensions as well! The standard ratio for most screens is 16:9 or 4:3.
In case the image doesn't look sharp, try with a higher resolution.
Ratios for images on different slide types:
Multiple choice - 16:9
Multiple choice (options) - 1:1
Word cloud - 16:9
Scales - 16:9
Open Ended - 16:9
100 points - 16:9
2 by 2 Grid - 16:9
Q&A - 16:9
Quiz - 16:9
Logo - 2:1
Full background image: 16:9