Image quality is bad

Hello,
I have a problem with the image quality. The image looks “not sharp”.
– 3 images in row are “not sharp”
example 300×180 https://www.ruff-bootsreisen.de/portfolio-item/hausboot-mieten-fluss-lot-frankreich/
– 4 images in row are “not sharp” 260px × 185px (Skaliert zu 302px × 215px)

What can I do to improve the images?
Happy you an improvement for all images of the website?
Regards
Rainer
PS. I read in the support forum about the function.php. But I didn´t know what can I do.
Settings WordPress
• Vorschaubild 80×80
• Mittelgroß 300×300
• Groß 1030×1030
Settings Enfold Theme functions.php:
$avia_config[‘imgSize’][‘widget’] = array(‘width’=>36, ‘height’=>36); // small preview pics eg sidebar news
$avia_config[‘imgSize’][‘square’] = array(‘width’=>180, ‘height’=>180); // small image for blogs
$avia_config[‘imgSize’][‘featured’] = array(‘width’=>1500, ‘height’=>430 ); // images for fullsize pages and fullsize slider
$avia_config[‘imgSize’][‘featured_large’] = array(‘width’=>1500, ‘height’=>630 ); // images for fullsize pages and fullsize slider
$avia_config[‘imgSize’][‘extra_large’] = array(‘width’=>1500, ‘height’=>1500 , ‘crop’ => false); // images for fullscrren slider
$avia_config[‘imgSize’][‘portfolio’] = array(‘width’=>495, ‘height’=>400 ); // images for portfolio entries (2,3 column)
$avia_config[‘imgSize’][‘portfolio_small’] = array(‘width’=>260, ‘height’=>185 ); // images for portfolio 4 columns
$avia_config[‘imgSize’][‘gallery’] = array(‘width’=>845, ‘height’=>684 ); // images for portfolio entries (2,3 column)
$avia_config[‘imgSize’][‘magazine’] = array(‘width’=>710, ‘height’=>375 ); // images for magazines
$avia_config[‘imgSize’][‘masonry’] = array(‘width’=>705, ‘height’=>705 , ‘crop’ => false); // images for fullscreen masonry
$avia_config[‘imgSize’][‘entry_with_sidebar’] = array(‘width’=>845, ‘height’=>321); // big images for blog and page entries
$avia_config[‘imgSize’][‘entry_without_sidebar’]= array(‘width’=>1210, ‘height’=>423 ); // images for fullsize pages and fullsize slider