To attach a file, simply open a composition or response window and drag your files onto the bottom area of that window.
Pictures can be either added in-line into the email (by dragging them into the email itself), or as an attachment (by dragging them into the bottom area)
— Alternatively, you can simply click the little paperclip icon in the bottom left and choose your file to attach.
— And, to choose the size of a photo you past inline, just click the “Insert Picture” button and choose what size you want.
Please note that you can now attach attach a file (either inline or as an attachment) when you are using the quick reply feature.