Ok there are many ways by which you can send photos to your friends in emails. Directly from your email services etc. but using your Mac account and sending the pictures can be directly done from the iPhoto app in Mac. Everything you have to do is to follow this guide -

Open iPhoto application in your mac. And then open the photo album view so that you can view many photos at the same time, than the enlarged single photo. You can directly open any web albums that you have created earlier.

Adjust any size of the photos, and make them according to your wish. This would set your image sizes. And now you can select the photo that you want to send. You can select multiple photos to send, but keeping the number limited to a smaller one would be better to have small attachment size.

After you have selected the photos, select the EMail option at the bottom right section of options.

You can select the size of the photos in accordance to Small, Medium and Large which may affect the size of the attachments. You also will have the options of including titles, comments.

And then you are taken to the Mail app in mac, with the attachments ready to be transferred and sent.

You can still change the image sizes in that mail compose box too.