Better image compression for thumbnails only

Hi guys,

it’s been quite some time since the start of my traveling blog in 2007. As at that time there were no real good image compression / cropping plugins, i always did my images manually with photoshop and upped them with ftp: thumbnails 150/150px, large images 1000/750px.

see: http://blog.ff-webdesigner.de/kitesurfen/langebaan-zum-zweiten.html

after ten years i needed a new repsonsive layout.
the start/category pages still got the same layout:
http://blog.ff-webdesigner.de/

the new blogposts feature a nice supersized full screen background slideshow.
all images are uploaded as real post attachments now.
http://blog.ff-webdesigner.de/kitesurfen/kitesurfing-israel-see-genezareth-sea-of-galilee.html

i got a problem with the thumbanils, drastically raising the page load.

i’m quite happy with the 200kb / full hd picture on the posts:
https://tools.pingdom.com/#!/dXZVO7/http://blog.ff-webdesigner.de/kitesurfen/kitesurfing-israel-see-genezareth-sea-of-galilee.html

but how can i minimize – and maybe also sharpen – the 150/150px thumbnail images in front / category pages? and JUST the thumbnails!
https://tools.pingdom.com/#!/b1UHfH/http://blog.ff-webdesigner.de/

55kb each for images that blurry in that size is miserable – used to be about 6kb each with manual editing.

i’m already using imsanity with 50% quality setting for jpgs – seems not to be affecting the generating of the thumbnails.

cheers
frank