If it's in JPG format you can right click it on your desktop (or where ever you have it) open with powerpoint picture manager. Click edit picture which will open a right side panel, click the link that says something like 'compress picture', that will prompt another series of links of which you will either check the one that indicates sized for email or sized for webpage. I prefer the latter.

Close the document, click yes when prompted to save, and your jpg will be substantially smaller.