Aqua Mail limits the data you download during sync, mostly to preserve your mobile data cap. This is less relevant than it was several years ago but everyone's situation is different so we leave it on by default.
To adjust these settings from default, you simply need to open an e-mail with linked/embedded images inside.
For Embedded images, tap the gear icon > Tap "Images and attachments" and choose how much information you want to preload from your embedded and attached files. Go back, tap the same gear icon, then "Message text" and do the same.
Note: Preloading this information means it will be downloaded during scheduled synchronization of your mail.
For linked images tap the gear icon and specify how you want to see images. You may choose to show images only from known contacts which you add to every time you allow an image or you can show images from everyone by disabling "Show only from known".