From the drop down navigation, select Profile, then click on the areas shown in the screenshot below to change either your Profile (avatar) or Cover Photo. 

Profile Photo (avatar)

Your Profile Photo may not exceed 500kb and must be square.  At time of upload, you will be given a chance to click and drag handles to adjust the portion of the image that will be displayed. 

Cover Photo

Your Cover Photo may not exceed 1mb and is displayed as a short, wide image.  After upload, you are given the opportunity to reposition the visible portion by clicking and dragging the image vertically.  You may reposition the Cover Photo in the future by clicking on the Cover Photo button, then selecting Reposition Photo.  Visitors to your Profile page may view the hidden portions of your Cover Photo by clicking on the image.

The screenshot below shows a completed profile page with both Profile and Cover Photos.

To add an image to the Members Gallery, click the + Create button at the top right of any page, then select Gallery Image from the drop down.  On the next screen, click Select then choose Member Albums and follow the prompts to add images to either a new album, your existing album(s) or directly to the Members Gallery.  When creating a new album, you have the option to allow comments and ratings for the images it contains.  You can change this at any time by going to the album, clicking Manage album then edit.

To view the album of a specific member, or to view your own album(s), visit the user's Profile page, then click the Albums tab.  To access all the images within an album, click on either the album title or one of the preview images from that album.

Why it happens

Short answer is because your phone was upside down when you took the picture. Your phone logs meta data in the photo indicating which direction you intended as up.  Unfortunately, this information isn't always transferred correctly when a photo is uploaded or emailed, and further, some source systems simply don't know how to read the orientation data. 

How to fix a photo after uploading to the forum

Open your photo in your editing software of choice and rotate the photo at least one full revolution, stopping when the orientation appears correctly, then save and upload it to the forum.  (e.g. if it looks right-side up on your device, rotate at least 1 full revolution before resaving. if it's upside down rotate it 1.5 times, so it is correctly oriented.) This works most of the time.  If, however, your photo is an exception, first try a different a different editing program if you have it (for Windows users, Paint seems to work best).  If you don't have another option or that still doesn't work, then a quick hack is to use the Snipping tool in Windows to take a capture of the photo while it is right side up or take a screenshot in Android or iOS and crop down to the image, then save and upload this new image to the forum.  

How do I avoid this from happening? 

Take your photos with the phone in it’s right-side up position.  In portrait mode, the lens should be at the top of the phone.  In landscape mode, the lens should be at the upper left corner.