To begin with, for any post you wish to embed images in, you'll need to first upload the images to an online hosting service or other web-attached storage service like a folder in your own hosting package if you own a domain name and have it hosted somewhere.
Also, the URL for any image you link to MUST end in an image format such as .jpg, .png, .bmp, or a similar supported image file format. Image links MUST NOT end in anything other than an image format, otherwise you must use a different method of linking, such as inserting "media" to link to a gallery of images rather than a single image.
For the average visitor I recommend using something like www.imgur.com or www.tinypic.com to host and link to images.
Once you create an account at one of these sites, you generally can save your uploaded images to your profile for future reference, but if the image is only likely to be referenced in this one post then simply uploading the image as a guest is sufficient. This can be done as follows.
Since MOST posted images will be of screenshots for diagnostic purposes, suffice to say that a screenshot can be obtained by pressing the windows key plus the print screen button and then navigating to the folder designated for saved images, at least with windows 8/8.1 and 10. This folder is generally located in the pictures library of your computer unless you've changed the designated folder to another location.
For other versions of Windows, additional steps may be required to take and save a screenshot and Google is a good resource for explicit instructions on how to accomplish that. For most newer BIOS screenshots you can simply press F12 and a screenshot of the current BIOS window will be created and can be saved to any usb flash drive currently attached to the system. Older non-UEFI based BIOS's are unlikely to have this feature. Check your motherboard manual or google your board model and BIOS screenshot to verify this feature.
In order to embed images in your post, you'll need a hosted link to refer to, so you can start by going to (For example) www.tinypic.com and clicking on the "Browse" button shown beneath the heading that says "Upload images and videos".
In the window that appears, navigate to the location of the image on your computer.
Click on the image and then click on "Upload now!"
On the resulting page there will be several fields listed.
Select the content in the field indicating "Direct link for layouts" and select Copy by right clicking on the selected text.
You are now ready to post the link into your thread. If you choose you can save this link to a notepad file for future reference or immediately use the link by clicking anywhere in the text field of the post you wish to include the image in, and then clicking on the "Insert image" button on the formatting toolbar as seen here.
In the box that opens you must paste your link and then click "Insert".
Click save after you've completed the rest of your post and you're done.
Alternatively, if you wish to post a link to a gallery of images, you can use the "Insert media" option on the formatting toolbar. There are other options available there as well such as "Insert quote" or "Insert spoiler", all of which are helpful tools and you can find more general information on these kinds of forum tools, again, with a quick Google search for the specific tool feature's name.
As a shortened version of the above procedure, you can post any hosted image into the post by simply adding the following code where you want it in your reply (Even quick replies), remembering to replace the highlighted text shown below with the actual link to your image.
Additionally, there are many other formatting options available on the dialogue toolbar including options for formatting text, inserting hyperlinks to other relevant data and presenting quote captions. Overall this is a highly useful way to maximize the effect of your post when necessary or desired. Of course, it can be overdone as well so it's recommended that you limit formatting options to those situations where it actually offers some benefit.