I need to get path of image which is located in www.mysite.com/themes/mytheme/img/image.png
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I've found the answer here: http://prestalab.ru/wiki/peremennye-v-shablonax.
The path to image in my theme will be: {$tpl_dir}img/myimage.png
$tpl_uri
will return the current theme's path relative to the web root.
Here are some other global variables PrestaShop makes available which may be helpful:
$img_ps_dir // the PrestaShop image directory
$img_cat_dir // current category's images directory
$img_lang_dir // current language's images directory
$img_prod_dir // current product's images directory
$img_manu_dir // current manufacturer's images directory
$img_sup_dir // current supplier's images directory
$img_ship_dir // current carrier's (shipping) images directory
$img_dir // current theme’s images directory
$css_dir // current theme’s CSS directory
$js_dir // current theme’s JavaScript directory
$tpl_dir // current theme’s directory (physical path on machine)
$tpl_uri // current theme's directory (web-root relative path)
$modules_dir // modules directory
$mail_dir // mail templates directory
$pic_dir // pictures upload directory