To find your previously created Midjourney images, there are several options depending on where you created them:
Discord
If you created your Midjourney images on Discord, you can access them by clicking on the search bar in the top right corner of the screen. Select "Mentions" and fill in your username, select it and hit "enter".
This will present you with a list of all the Midjourney images you have made in the past. To obtain the highest resolution version of any image, simply click on the desired image and proceed to download it.
You can also start a private chat with the Midjourney Bot so you don't get lost in the endless messages stream.
Midjourney Web App
You can also use the Midjourney web app to see all of your images. This shows only your upscaled images, which is quite useful. I'm not sure but I think this goes only for paid accounts.