Creating new PRTG icons is very simple. The important thing is to follow these guidelines.
To edit the files you can use any graphics software for resizing and that exports the images to PNG.
You can find the logos of various vendors on Google.

The vendors icons are located in here: //SERVER/c$/Program Files (x86)/PRTG Network Monitor/webroot/icons/devices

You have to create PNG files with a standard size of 14×14 (8 bits depth).
Then copy them to the folder specified above.
They will immediately appear in PRTG under settings, where you choose the vendor icons, and they will be in alphabetical order.

OPTIONAL BUT PRACTICAL:
Use the vendors_VendorName.png nomenclature.