You can put images directly on your dashboards in Grow, but you need to have the image URL. (The article about how to create an image metric is here.)
Luckily there are a few quick and free image hosting services.
Note: Image hosting services have widely varying terms of service, types of content, and private/public policies. Make sure you are aware of what images your image could be shown with, how they can and will use your image etc. When in doubt, don't upload sensitive images.
Here are the step by step instructions for FunkyImg.
1. Drag your image to the upload box
2. Click the 'Upload imgs!' button
3. Copy the 'Direct Link' URL
4. Paste it into the 'Image URL' field
5. You're done! Save your metric.
If you have any questions please email firstname.lastname@example.org or chat in on the site.