>>203535871000 hours in MS PaintFirst off, I used a Chrome extension that captures the whole page. Then I opened it on paint and cropped the top and bottom of the thread that was unnecessary, to only leave the OP and the posts in the picture.
I then started selecting the unnecessary posts to the capture, and pressing the Del button to get rid of them. This leaves a white space where the posts were; all you have to do is use the Color Picker tool in Paint, take the background color of 4chan's page, then use the bucket to fill in those white spaces that are left from removing the unnecessary posts.
After that, you should have an image with only the posts you want, but some of them may be separated by long blank spaces. To fix this, you simply cut out the posts that are separated from the others (making sure you cut them out while still leaving that small blank space to the left of posts; in other words, cut said posts starting from the very left edge of the picture), and paste it close to the posts that are united, then using the directional arrows of your keyboard to adjust the pasted image to be perfectly aligned with the rest of the picture.
Repeat the above until you have all the posts without those blank spaces between them. Now, just cut out whatever remains of the picture that isn't the posts, and voilà.