Ekki eyða myndinni; ég þarf skjámynd af síðunni líka.

In Icelandic, negative commands are usually formed with ekki + infinitive.

So:

Ekki eyða myndinni = Don’t delete the picture

A positive command would normally use the imperative:

Eyddu myndinni = Delete the picture

So eyða stays in the infinitive here because the command is negative.

Because eyða is a verb that takes the dative case in Icelandic.

That means the thing being deleted is put in the dative, not the accusative.
So myndinni is the dative singular definite form of mynd.

Very roughly:

• mynd = picture
• myndin = the picture
• myndina = the picture, accusative
• myndinni = the picture, dative

Since eyða governs the dative, myndinni is the correct form.

In Icelandic, the definite article is usually attached to the end of the noun instead of appearing as a separate word.

So:

• mynd = picture
• myndin = the picture

When the noun changes case, the article changes with it and merges into the ending:

• myndinni = to/from/on the picture depending on context, here simply the picture in dative
• síðunni = the page in dative

So there is no separate word like English the here; it is built into the noun ending.

Icelandic has no indefinite article like English a/an.

So a bare noun can mean a screenshot:

ég þarf skjámynd = I need a screenshot

If the sentence meant the screenshot, Icelandic would mark that on the noun itself, for example skjámyndina in this context.

So skjámynd is indefinite here: a screenshot.

The verb þurfa normally takes an accusative object:

ég þarf skjámynd

But many feminine nouns in Icelandic have the same form in nominative and accusative singular.
So even though skjámynd is the object and is in the accusative, it looks unchanged.

That is very normal in Icelandic.

It is a compound word:

• skjár = screen
• mynd = picture/image

So skjámynd literally means something like screen-image, which is the normal Icelandic word for screenshot.

A useful extra point: in Icelandic compounds, the last part usually determines the gender, so skjámynd is feminine because mynd is feminine.

Because the preposition af takes the dative case.

So síða becomes síðunni here.

This phrase:

skjámynd af síðunni

means a screenshot of the page.

Depending on context, af can often feel like of or from in English, but with skjámynd the natural translation is of the page.

mynd is a general word for picture, image, or photo.

skjámynd is specifically a screenshot.

So the sentence is talking about two different things:

• myndinni = the picture/image that should not be deleted
• skjámynd af síðunni = a screenshot of the page that the speaker also needs

líka means also, too, or as well.

Putting it at the end is very natural in Icelandic:

ég þarf skjámynd af síðunni líka = I need a screenshot of the page too

It shows that this is an additional thing the speaker needs.

You could also say Ég þarf líka skjámynd af síðunni, but the emphasis changes slightly. The version in your sentence sounds very natural and conversational.

The semicolon links two complete clauses that are closely related:

• Ekki eyða myndinni
• ég þarf skjámynd af síðunni líka

It shows a stronger break than a comma, but a closer connection than a full stop.

So the punctuation helps show the logic:

Don’t delete the picture; I need a screenshot of the page too.