The file and directory structure of a Payage gateway addon is as follows.
com_payage_paypal_1.00 Top level addon directory
payage_paypal_lesarbres.xml The Joomla installation manifest file
paypal_lesarbres.php The model, which contains all of the gateway code
paypal_lesarbres.xml JForm file for the account configuration page
language Directory containing the language files, if any
en-GB Each language must have its own directory
en-GB.com_payage_paypal_lesarbres.ini The English language file
pay_buttons Directory containing payment button images
paypal_ft200.png Framed, Transparent, 200px x 100px
paypal_fw200.png Framed, White background, 200px x 100px
paypal_ut200.png Unframed, Transparent, 200px x 100px
paypal_button_grey_200.png Other useful buttons can be included
paypal_button_grey_128.png
icons Icons should be 36px high
paypal_black.png Black is always supplied
paypal_white.png White is always supplied
paypal.png A colour icon is usually supplied