Dear partners
I hope you are well! i need some more help and info about my website. The photos are not so clear although the size and resolution are appropriate according your instructions.
For the K2 items the size of the photos is 1320 x 740 and the image size settings are XLarge
The slide show in the first page is constructed with K2 items
The photos size in joomla articles is 760 x 400 and the nucontent settings XLarge

K2 item: kentromeletis.edu.gr/index.php/m-about-us-home/m-about-us
Joomla article: kentromeletis.edu.gr/index.php/m-km-ann/41-prosklisi-symmetoxis-sta-epanaliptika-themata-oefe

www.kentromeletis.edu.gr

The XL image size should be about 1000 - 1300 pixels in width, with the best resolution for quality being at 1320, the same as the maximum size width.

The current resolution of 760 is too small, you need to increase that.
The tricky part now is resizing the already existing images.

You can reupload these images or try looking for plugin at the K2 Community.
there was a free one but unfortunately I cannot recall its name (it must be on gihub)
getk2.org/community

Ok for joomla articles but for the K2 items the size is already: 1320 x 740

I mean the XL image size in your K2 params.
You may be uploading them at 1320 but they are scaled down and then stretched (hence the blurring).

Be sure to double check that category's settings as well to make sure you are not using any overrides.

ok i thing is better now for K2 items!

I 'm glad it worked :)