The email size issue is not about the size as in 100px by 100px (although your image should be sized to fit within the template). It's about compressing the file so the file size is smaller.
Image editing software such as Adobe Photo Shop or Picasa or a free online service like Pixlr (http://pixlr.com/), tiny png (https://tinypng.com/) or Website Planet (https://www.websiteplanet.com/webtools/imagecompressor/)can be used to resize the image. Another commonly available program is Microsoft Office Picture Manager.
To compress your images using Microsoft Office Picture Manager:
- Open up your images in Microsoft Office Picture Manager.
- From the Picture menu, select Compress Pictures.
- On the right side of the screen you'll see the compression options. Under Compress for:, select Email messages.
- The Estimated total size of the Original and the Compressed image will be displayed.
- Select OK to save.