image editors start with a canvas; canvases can be sized and filled with different backgrounds or
no background at all
72ppi is the best practice resolution setting for web display
image editing often involved layers of content. Layers can be reordered or stacked in a different order to
affect the composition of more complex images
when saving your work in an image editor (like GIMP), save an editable version that can be modified later.
export a flat file--a .jpg, .gif, .png--for use in your website
do not keep your editable files in your web structure--these will not render; only keep your exported flat files in the website structure
an alpha channel will add a layer of transparency to your image