but it still doesn't show a picture in the activity stream

It displaying images depends on how you've created the article. If the images are directly in the article they will display fine. If they're under the "Images and Links" tab they won't display as that's custom behavior for the content component that we don't replicate in the activity (but likely will in a future release). Below is what the article activity will look like. You can see in my previous reply a screenshot where I've directly inserted an image into the article which displays fine.

and it doesn't cut the text.

It's not large enough to cut the text. You should however be cutting the text yourself in your articles using a read more break.

Any way I can set it to show only a thumbnail of it?

All it does is render the article as you've input the article. So to limit image size you'll need to either use custom CSS or edit the article and limit it on the image in the article with inline CSS or sizing attributes. Your WYSIWYG editor should have options for that. If you want to add custom CSS you can input custom CSS in CBs built in CSS overrides functionality below.

www.joomlapolis.com/blog/kyle/18711-template-css-overrides-made-easy

Now i have the read-more, but: the read more only expands the post - can I make it so it opens the article?

No, clicking the title opens the article. That read more built into CB Activity is just a means of preventing massive posts from pushing everything down the page.

Also: can I edit the layout? since movie posts like this with posters have portrait images (the posters), it would make sense to show the text next to the image.

Yes, the activity output is entirely template driven. You'll find the template file below.

components/com_comprofiler/plugin/user/plug_cbactivity/templates/default/core/display/article.php

Making changes to default template will cause your changes to be lost on update. So what you'll want to do is create a new folder in the templates folder then copy of the template you want to edit. Now set CB Activity > Parameters > Global template to be your folder. So you'd have the below for example for your custom template.

components/com_comprofiler/plugin/user/plug_cbactivity/templates/custom/core/display/article.php

Now you can edit article.php and custom the article activity display to whatever you like. This will be improved further in CB 3.0 when we replace most template output with VueJS views and Twig template files, but for now it's mixed PHP and HTML.