From the desktop site:
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.
From the iPhone app:
Tap Profile in the navigation bar. On the next page, tap on your profile photo, or hit Edit in the top right corner of the screen. From here, click change. You'll have the option to upload a photo from your phone's photo gallery, or to take a new photo.
From the Profile tab, tap 'Edit profiles.' Swipe to the group that you'd like to change your profile photo for, and tap the photo. Then, select 'Upload' or 'Camera.' Find the photo you'd like to use, then tap 'Save.'
Please note that it is not currently possible to delete a profile photo entirely through the app. To remove a profile photo entirely, you'll need to log in to the full site and visit your profile page where the photo appears.
From there, click 'Photos' below your profile photo. Navigate to the picture you'd like to remove, and then click 'Delete.'