Thumbnails issue / Force images size issue

-- HikaShop version -- : 2.4.0
-- Joomla version -- : 3.4.0
-- PHP version -- : 5.4.27
-- Browser(s) name and version -- : Firefox 36.0.1
-- Error-message(debug-mod must be tuned on) -- : Error: No se pudo escribir en la carpeta /home/xxmyuserxx/public_html/catalogo/media/com_hikashop/upload/thumbnails/320x0f

Hello: I explain certain circumstances that would like some confirmation or correction of my conjectures.

Intro: In most of my products I use variants, and one or more images for any variant; the "Product page image height" is set to 320px and the width is not defined in order to preserve proportions.

The issue: In product view of the front-end I got the following error message:
"Error: No se pudo escribir en la carpeta /home/xxmyuserxx/public_html/catalogo/media/com_hikashop/upload/thumbnails/320x0f"
what means the system is unable to write the folder where the thumbnails should be created.

First of all, I checked for permissions via FTP: all folders have 755 permissions and files have 644 (I tried changing to 775 but the error persists). Also, I tried deleting the issue folder: the system recreated it (as it is normal), but the error message reappeared.

I noticed (via FTP) that some of the thumbnails for the product variants were recreated in that folder (as it is expected) EXCEPT the images that are smaller than the intended output. So I tried unchecking "Force images size" (in Hika>System>Config>Main>Images) and get no error.

Ask: Does this mean that the system is not able to resize small images to a larger size view?
Should the "Force images size" add some white (or black) margin to smaller images? What is the aim of this option if not?

(All of this doesn't seem an "issue" but is not obvious what happened in my case; I've thought if someone get the same message this forum could be useful)

Regards,

1 thing that could be mis-configured is your ownership not just your permissions settings.

on ubuntu type: sudo chown -R root:www-data ./

make SURE you are on the root directory of the web host!! usually thats /var/www/sitename.com on most apache installs.

i'm no hikashop expert, I have a hikashop question for you. I have been trying to locate anywhere i could change the size of the products page thumbnails. without luck. modules and menu items were a breeze ..

Help!! lol

Thanks Nicolas, now it's fixed following your indications (altough, note that resizing images to a mininum of 320px* (* in my case) the error messages dissapeared.

Thanks @shady717, but in this case is nothing to do with permissions or ownership. I ignore why the message say that I was not able "to write" the folder. Anyway, the issue was not indicating both, the height and width.