Cannot attach photos to my Yahoo or Gmail messages

Hi apspectator,

Welcome to the Apple Support Communities! Based on your description, you have been unable to send photo attachments within the Mail app since updating to El Capitan. I know how much I rely on being able to utilize mail attachments on a daily basis, and there are definitely some troubleshooting steps we can walk through to address this for you.

To begin, check out this support article for some general troubleshooting suggestions:

If you can’t send or receive email on your Mac

Specifically, take a look at this section:

If you still can’t send or receive email

1. Make sure that you installed the latest Mac software updates, especially if the issue occured immediately after installing a previous update.
2. In OS X El Capitan, you might see a status icon and brief error message in the upper-right corner of the Mail window, beneath the Search field. The message might say “Network Offline” or “Login Failed,” for example. Click the message to see more details about the issue.
   
3. Check your connection in Mail Connection Doctor. It might be able to tell you more about the issue.

Next, this article has some helpful recommendations for issues specific to El Capitan:

Mail (El Capitan): If you can’t send messages

Finally, this resource has a basic overview of adding attachments within El Capitan. This includes utilizing Mail Drop for large attachments. Given the large file size for most photos, that could be what you're seeing:

Mail (El Capitan): Add attachments

Send large attachments using Mail Drop

You can use Mail Drop to send files that exceed the maximum size allowed by your email account provider. Mail Drop uploads large attachments to iCloud, where they’re encrypted and stored for up to 30 days.

If you have an iCloud account and you’re signed in to iCloud when you click Send, Mail automatically sends the attachments using Mail Drop. Mail Drop attachments don’t count against your iCloud storage.

If you don’t have an iCloud account, or if you’re not signed in, Mail asks you whether to use Mail Drop (to always use Mail Drop select “Don’t ask again for this account”).

If a recipient uses Mail in OS X Yosemite or later, the attachments are included in your message. For other recipients, your message includes links for downloading the attachments, and their expiration date.

You can turn Mail Drop on or off for an account. Choose Mail > Preferences, click Accounts, select the account, click Advanced, then select or deselect “Send large attachments with Mail Drop.”

All the best!