The methods vary slightly depending on which phone you are using (and even which messaging app you choose if you install a different one) but, for the most part, there are two main ways to do this. The one that should be most consistent across all Android devices and messaging apps is to find the picture in your gallery (or whichever photo browsing app you use) and select it. You should see a share button at the top of the picture (tap the pic if you don't) which kind of looks like a wishbone. This will ask you which app to use, scroll through the list until you find your messaging app and select it. It will then prompt you to choose a conversation or select a new one. Once this is done, press send.
The other method is direct from the messaging app. You should see a paperclip next to the text box. Press this, and it will take you to the gallery to select the picture.
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