To insert an SVG image into the signature, just go to the Insert tab in the Ribbon -> Pictures -> Pictures. Using them here ensures they display at the correct size later. Outlook will convert them to PNG, but that's OK. For any images you'll have in your signature, in the Outlook message where you're creating the signature, use the SVG images you will ultimately want to store. Here's a step by step to do this in 2022 (note that this comes from a response I wrote to a newer version of this same question at. Then you just go into the Signatures folder where all of these are saved and replace the PNG files with SVG.

Outlook will convert these to PNG files without telling you (presumably for better compatibility). The trick is to create the sig in the Outlook message using the SVG images and size everything the way you want in the original message before copying and saving as a signature. Assuming you already have a sig designed, it's a very quick (about 1 minute) process to convert to SVG images.

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