You can upload a photo to your Meetup profile through your General Account page here.
Once you've added a main profile photo, that photo will be your default photo for any new Meetup Groups you join.
To change your profile photo for a specific Meetup Group, head to that Meetup Group's home page, and scroll over My profile in the top navigation bar. From there, select View profile where you'll find the option to Change your photo. Click there to upload a new photo or choose from your existing photos.
You can also choose to import your profile photo from Facebook.
Note: Photos uploaded to meetup must be in JPEG, PNG, or GIF format and under 10MB.