Nítján myndir eru í möppunni minni, og tuttugu aðrar eru á minnislyklinum.

Because the sentence is talking about more than one picture: nítján means nineteen, so the noun has to be plural.

• mynd = picture, image, photo
• myndir = pictures, images, photos

Here myndir is the nominative plural, because it is the subject of eru.

Because the subject is plural.

• er = is
• eru = are

Since nítján myndir means nineteen pictures, Icelandic uses the plural verb: myndir eru = pictures are.

The same happens in the second clause:

• tuttugu aðrar eru = twenty others are

Not in this sentence. Numbers like nítján and tuttugu are normally used in a fixed form.

That is different from lower numbers like one, two, three, four, which often do change for gender in Icelandic.

So here you simply get:

• nítján myndir = nineteen pictures
• tuttugu aðrar = twenty others

The noun after these numbers is plural.

Because Icelandic, like English, can leave the noun out when it is obvious from context.

So:

• tuttugu aðrar literally means twenty others
• the understood noun is myndir

A fuller version would be:

• tuttugu aðrar myndir eru á minnislyklinum

But once myndir has already been mentioned, leaving it out sounds natural.

Aðrar is the feminine plural form of annar, which means other or another.

It has to match the missing noun myndir, because mynd is a feminine noun.

So the agreement is:

• myndir = feminine plural
• aðrar = feminine plural

If the noun were masculine or neuter, the form would be different.

Because the prepositions í and á take the dative when they describe location rather than movement.

Here the pictures are already located somewhere:

• í möppunni minni = in my folder
• á minnislyklinum = on the USB drive

That is static location, so Icelandic uses the dative.

A very important contrast is:

• í möppunni = in the folder
• í möppuna = into the folder

and

• á minnislyklinum = on the USB drive
• á minnislykilinn = onto the USB drive

So:

• dative = being in/on a place
• accusative = moving into/onto a place

Because Icelandic uses different prepositions depending on how something is conceptualized.

• í usually means in
• á usually means on

So:

• í möppunni = in the folder
• á minnislyklinum = on the USB drive

With digital storage, Icelandic often uses á for things stored on a device or medium. That may feel natural to English too, since we also say files are on a USB drive.

Because in Icelandic, possessives like minn / mín / mitt very often come after the noun when the noun has the attached definite article.

So the normal pattern here is:

• möppunni minni = the folder of mine / my folder

This is a very common structure in Icelandic:

• bíllinn minn = my car
• húsið mitt = my house
• möppunni minni = in my folder

So the noun comes first, then the possessive agrees with it in gender, number, and case.

Because the possessive has to agree with möppunni.

The noun mappa is:

• feminine
• singular
• here in the dative

So the possessive must also be feminine singular dative:

• mín = feminine singular nominative
• minni = feminine singular dative

That is why you get:

• í möppunni minni

not

• í möppunni mín

Möppunni means the folder, in the dative singular.

It comes from mappa = folder.

The important things happening here are:

1. the noun is in the dative because of í with location
2. the noun is definite: the folder
3. Icelandic usually adds the as an ending on the noun

So:

• mappa = a folder
• möppunni = in the folder / the folder (dative form)

This kind of attached definite article is extremely common in Icelandic.

Minnislyklinum is the dative singular definite form of minnislykill, which means USB flash drive or memory stick.

Literally, minnislykill means something like memory key.

In this sentence:

• á minnislyklinum = on the USB drive / on the memory stick

Again, the form is dative because á is describing location, and it is definite because it means the USB drive.

Yes. Mynd is a broad word.

Depending on context, myndir can mean:

• pictures
• images
• photos
• even films/movies in some contexts

In this sentence, because the pictures are in a folder and on a USB drive, many learners will naturally understand it as images/photos/files. The exact English choice depends on context, but the Icelandic word itself is flexible.