Can I pray in clothes with pictures on them?

Yes, your prayer is still valid in a shirt with pictures on it, as long as the clothes are clean and cover what they should. But images of people or animals, offensive slogans or religious symbols are best avoided in prayer, and many scholars call praying in them disliked (makruh).

Is the prayer valid?

For a prayer to count, your clothes need to be pure (free of najasah) and cover your awrah. A print on the fabric doesn't change either of those. So if you prayed in a graphic tee before you knew about this, you don't need to repeat anything. See What do I have to cover when praying at home alone?

Why pictures can be a problem

Two hadiths explain the concern:

The Prophet ﷺ prayed in a garment with patterned markings, then said it had distracted him and asked for a plain one (Sahih al-Bukhari 373).
Aisha had a curtain with pictures in her room, and he asked her to remove it because its images kept appearing to him while he prayed (Sahih al-Bukhari 374).

So even simple designs can pull attention away from Allah. Images of living beings (people, animals) carry an extra concern, because the Prophet ﷺ discouraged displaying them, and angels don't enter a house with such pictures. See Can I make AI images of people as a Muslim?

What scholars say about different prints

Images of people or animals: praying in them is makruh according to all four schools, though the prayer counts. Some Hanafis say it is less of an issue if the image is small or the face is unclear.
Crosses, idols or symbols of other religions: avoid wearing these at all, not only in prayer.
Rude words, alcohol or band logos with bad messages: not fitting for standing before Allah.
Landscapes, flowers, stripes, text like a college name: generally fine, though a plain garment is still better for focus.

Dress well for prayer

Allah tells us to take our "adornment" for prayer, meaning to dress decently and nicely:

يَـٰبَنِىٓ ءَادَمَ خُذُوا۟ زِينَتَكُمْ عِندَ كُلِّ مَسْجِدٍ "O children of Adam, take your adornment at every masjid." Quran 7:31

You don't need special Islamic clothing. A clean, plain T-shirt and trousers are perfect. See Can I pray in jeans?

What if I have nothing else?

At work, school or when traveling you may only have the shirt you're wearing. Pray anyway: the prayer on time matters far more than a print. If the image is of a person or animal, you can turn the shirt inside out, cover it with a jacket or scarf, or fold the area away so it isn't visible.

Key points

Prayer in printed clothes is valid if they are clean and cover the awrah.
Images of people or animals are disliked in prayer.
Religious symbols and offensive messages should be avoided.
Plain clothes help your focus, following the sunnah.

Tip: Keep one plain, clean top or a light zip-up hoodie in your bag or desk drawer. You can slip it on over any outfit for a quick prayer at work or university.